1. Field of the Invention
This invention generally relates to an apparatus for reproducing information from a recording medium such as an optical disc. This invention specifically relates to an information reproducing apparatus including a waveform equalization circuit for processing a reproduced signal of a run-length-limited code. In addition, this invention relates to a decoding apparatus.
2. Description of the Related Art
Japanese patent application publication number 10-106161/1998 discloses an optical information reproducing apparatus based on a PRML (partial response maximum likelihood) system. In the apparatus of Japanese application 10-106161, information of a run-length-limited code is reproduced from an optical disc through a reproducing section, and a transversal filter subjects the reproduced waveform to partial-response equalization depending on tap coefficients. The output signal of the transversal filter is decoded into binary data by a maximum-likelihood decoder. A parameter setting device selects intersymbol-interference imparting values in the partial-response equalization in accordance with the characteristics of the reproduced waveform. Also, the parameter setting device sets the tap coefficients of the transversal filter and a decision point signal level for the maximum-likelihood decoder as parameters in response to the selected intersymbol-interference imparting values.
In the apparatus of Japanese application 10-106161, the parameter setting device includes a memory loaded with parameter-setting reference data. A target after-equalization waveform is determined on the basis of the parameter-setting reference data and the selected intersymbol-interference imparting values. The apparatus of Japanese application 10-106161 premises that the optical disc has predetermined pits (reference pits) representative of the parameter-setting reference data. A reference-data signal is reproduced from the predetermined pits in the optical disc. The tap coefficients of the transversal filter are set to equalize the waveform of the reproduced reference-data signal to the target after-equalization waveform.
In the apparatus of Japanese application 10-106161, the parameter setting device includes an error-rate deciding device which compares the binary data outputted from the maximum-likelihood decoder and the parameter-setting reference data fed from the memory to calculate the rate of bit errors therebetween. The error-rate deciding device judges whether or not the calculated bit error rate is within an allowable range. The selection of intersymbol-interference imparting values are responsive to the result of the judgment by the error-rate deciding device. The tap coefficients of the transversal filter and the decision point signal level for the maximum-likelihood decoder which occur when the calculated bit error rate is within the allowable range are actually used in the partial-response equalization and the decoding procedure.
The apparatus of Japanese patent application 10-106161 fails to implement suitable waveform equalization for a signal reproduced from an optical disc which lacks predetermined pits representative of parameter-setting reference data.
Japanese patent application publication number 7-192270/1995 discloses an apparatus for reproducing a digital signal of a run-length-limited code from an optical disc. The apparatus of Japanese application 7-192270 uses a method suited for a high information recording density. The method in Japanese application 7-192270 performs ternary equalization whose objects are only an amplitude except for points corresponding to a data train provided with a minimum code inverting gap among points just before or just after the inverting position of a code and an amplitude at the inverting position of the code.
In the apparatus of Japanese application 7-192270, a signal is read from an optical disc by an optical head, and the read signal is applied through an amplifier to an equalizer. A decider following the equalizer discriminates the level of the output signal of the equalizer. The decider includes two comparators. The output signals of the comparators are fed to an error calculation circuit as level discrimination results. Since the decider includes the two comparators, the signal processing by the decider is relatively complicated and the level discrimination results provided by the decider tend to be adversely affected by noise and signal distortion.